Transmission control circuit



Jan. 5, 1932.

fa. s. BJORNSON TRANSMISSION CONTROL CIRCUIT FIG. 3

Filed Sept. 23, 1930 lNVE/VTOR I aaBJoRA/soN AT RNEY Patented Jan. 5, 1932 UNITE 1 m i BJORN e. BJoRivsoN, on NEW YORK, n.1,

PATENT. ()FFICE ASSIGNOR TO BELL TELEPHONE LABORA- TORIES, INCORPORATED, OF NEW YORK, Y.-, A CORPORATION OF NEW YORK:

TRANSMISSION CONTROL CIRCUIT Application filed September 23, 1930. Serial ma. 483,784.

This invention relates to signal transmission circuits ofthe four-wire type and particularly to the signal-controlled means employedtherewith for suppressing echoes and 5 preventing slnging.

An object of the inventionis to improve the operation Characteristics of long fourwire signaling systems, employing signal= controlled apparatusfor suppressing echoes m and preventing singing. e

To obtain proper operationof long tourwire telephone systems, it has been .found necessary in the past to utilize-in connection therewith signal-controlled apparatus for effectively disabling one ofthe associated two-wire paths while signal transmission is taking place over the other, so as to prevent echo currents or reflected currents from being transmitted back to t-he transmltting end of v the system and causing either a disturbance.

or singing. The disabling apparatus usually comprises meanasuch asan amplifier-rectitier control circuit, for diverting a portion of thesignaling currents from one path and utilizing it to control the open-circuiting or short-circuiting {of the oppositely directed path, or to control the operation characteristics of an amplifier in the oppositely directed path.

It is the usual practice to arrange these signal-controlled devices, or echo suppressors as they are commonly called, one of which is associated with each two-wire line, at some intermediate station, for example, at a mid-v way point. Such an arrangement has a dis? advantage in that it may prevent the sub- ,scriber'who desires to talk in one direction a from getting immediate control of the circuit after it has been used by a subscriber talking 40 in the other direction. This is due to the fact that, owing tothe time interval which it takes for a wave to be transmitted from the echo suppressor at one side of the line through the terminal to the point at which the opposite line is disabled the echo suppressor must be provided with hang-over means to main tain the line disabled for a certain length of time after thefirst talker has finished talking) In another arrangement of the prior art this difiiculty has been eliminated by locating the two unitsof the echo suppressor respectively at or near the ends of the fourwire circuit. This latter arrangement, however, has a disadvantage in that transmission lockouts may occur When the two subscribers at opposite ends of the system both start talking at about the same time, due to the simultaneous lockingup of the twosuppressor units. I

' The disadvantages mentionedabove of the circuits of'the prior art are eliminated in the circuit of the present invention by utilizing in. combination with each suppressor unit for disabling one two-wire signaling path when signals are being transmitted over the other oppositely directed two-wire path, a circuit responsive to the transmission of signals over thefirstpath for disabling theecho suppressor unit, but with a time delay of substantially the same order of magnitude as the overall transmission time of the four-i wire circuit between terminals; In the system of the invention, when the subscribers at opposite endsof the system both start talking within a time interval equal to the overall transmission time of the four-wire circuit 7 between terminals, complete control is obtained by the first'talker, because, when an echo suppressor unit becomes operative before the associated echo suppressor disabling circuit,th-e disabling process is arrested. The necessity for utilizing appreciable hang-over in the echo suppressors is eliminated by logrid potential toa vacuum tube amplifier in one two-wire path which is directly proportional to the amplitude level of the waves transmitted over the other path, so as to prevent the locking up of the circuit by noise while allowing operation of the echo suppressor on quiet lines at higher sensitivity, and each echo suppressor disabling circuit comprises an amplifier-detector circuit controlling a chain of mechanical relays in which the desired relay is produced by proper design of the operating and release times of the relays.

The objects and advantages of the invention will be better understood from the following detailed description thereof when read in connection with the accon'ipanying drawings in which Fig. 1 shows schematically a four-wire toll telephone circuit equipped with transmission control circuits in accordance with the invention;

. Fig. 2 shows in greater detail a preferred construction of the transmission control circuits utilized in the system of Fig. 1; and

Fig. 3 shows a modification of a portion of the circuit of Fig. 2 which may be used as an alternative.

The diagram of Fig. 1 is not an actual circuit diagram but rather a single-line layout, each line indicating a transmission path. A normal break in a path is indicated by separated arrow heads; a normal make by contacting arrow heads. To show that a transmission element is to be varied by a device an arrow head points from the device toward the element. An arrow directed at a make point indicates that the path will be disabled at that point.

The four-wire telephone circuit of 1 comprises a one-way transmission path EA including the one-way amplifying devices A and A for repeating telephonic currents in the direction from west to east between the I westtwo-way line section L and the east two-way line section L and the one-way transmission path WA including the oneway amplifying devices 1 3 and A, for repeating telephonic currents in the direction from east to west between the east two-way line section L and the west two-way line. section L In well known manner the oppositely directed one-way transmission paths EA and WA may be connected in substantially conjugate relation with each other and in energy transmitting relation with the west line section L and the east line section L respectively for example, by hybrid coil transformers and associated balancing networks or other suitable means (not shown).

Bridged across the input of amplifier A near the output of the path EA is the input of an echo suppressor unit ES operating in response to the waves transmitted over the path EA to adjust the gain of the amplifier A near the input of the path WA, as indicated by the arrow 1, so as to introduce in the path WA a loss which is directly proportional to the amplitude level of the waves from the part EA impressed upon the input of the echo suppressor E8 Similarly, bridged across the input of the amplifier A. near the output of the path W'A is the input of an echo suppressor unit E8 operating in response to the waves transmitted over the path WA to adjust the gain of the amplifier A near the input of the path EA, as indicated by the arrow 2, to introduce a loss in the path EA which is directly proportional to the amplitude level of the waves from the path WA impressed upon the input of the echo suppressor ES Connected across the output of the amplifier A near the input of the path WA is the inputof a device D which will be referred to hereafter as the disabler D which operates in response to the waves impressed thereon from the path WA, but with a given time delay, to effectively disable, as indicated by the arrow 3, the output of the echo suppressor E8 for example, by opening the normally closed contacts 4 therein. Similarly, connected across the output of the amplifier A near the input of the path EA is the input of a device D which will be referred to hereafter as the disabler D which operates in response to the waves impressed thereon from the path EA, but with a given time delay, to disable the output of the echo suppressor ES as indicated by the arrow 5, for example, by opening the normally closed contacts 6 therein. The time delay in the operation of the device D in response to the waves from the path WA, and the time delay in the operation of the device D in response to the waves from the path EA, is preferably substantially equal to the overall transmission time between the line sections L and L over one side of the four-wire circuit EA and WA, respectively.

The preferred construction of the echo suppressors ES and ES and the disablers D and is shown in detail in Fig. 2. As indicated in Fig. 2, the amplifier A in the path EA preferably consists of two or more stages, the first stage comprising the threeelectrode vacuum tube amplifying devices 7 and 8 connected in push-pull relation. The echo suppressor ES comprises the one-way amplifying device A having its input connected across the path WA in the input of the amplifying device A. a full wave vacuum tube rectifier R having its input con- 'nected to the output of the amplifying device A through the transformer 9, and a network 10 comprising the resistance 11 and parallel condenser 12, connected in the common grid circuit for the push-pull tubes 7 and 8 in the first stage of the amplifier A in the transmission pat-h EA. \Vaves impressed on the input of the suppressor E8 will be amplified by the amplifying device A and then rectified by the full wave vacuum tube:

' rectifier R The rectified output of the rectifier R willbe impressed on the input circuits of the tubes 7 and 8 in the push-pull stage of the amplifier A across the parallel resistance 11 and condenser 12 causinganegative biasing potential to be applied in common to the grid of the tubes 7 and 8 which is directly proportional to the level of the waves in the input oft-he echo suppressor ES This negative biasing potential ape plied to the grid of the tubes 7 and 8 in the amplifier A will effectively vary the gain of the amplifier A to insert a loss in the path EA which is directly proportional to the a1nplitude level of the waves transmitted over the path WA. The amplifiers A and A are so designed'that when the associatedsuppressor 1s short clrcuited in its output the amplifier will repeat with maximum efficiency.

The purpose of the condenser 12 in network 10 is to smooth out the output of the rectifier R so as to prevent sharp changes in the negative biasing potential applied to the grids of tubes 7 and 8. i

A loss in the path EA which is proportional to the amplitude level of the waves transmitted over the path WA could be pro'-:

duced in other ways than that. shown. For example, the rectifier R could be made to control the variable plate-filament impedances of vacuum tubes placed in series with or in shunt across the path WVA, inaccordance with the amplitude level of the'waves in the path EA. A t I As indicated in Fig.2 the disabler D may comprise the one-way amplifyingdevice A having its input connected across the path EA in the output ofthe amplifying device A a vacuum tube rectifier R having itsin put connected to the output of the amplifying device A through the transformer 13, and

'a slow-operating, slow-releasing relay circuit comprisingthe chain of'relays'l l, 15

and 16 controlled by operation ofthe recti fier R g Therelay 14 is connected directly in the output? circuit of the rectifier R and is responsive to operation of "that relayto open its normally closed contacts 17 to break the energizing circuit for slow operatingrelay 15 which is normally energized by cur-' The relay chain comprisingthe relays 14,

15 and 16. in the disa-bler D is designed so echo suppressor ES at the points 22 in re-- sponse to the west to east. speech waves transmitted over the path EA, a given time. after these Waves areiinpressed on the input of the amplifying device'A and so as to maintain the echo suppressor ES short-circuited for a given hangover time after the supply of: the west to east speech waves to the input of the amplifying device A ceases. The time delay operation of the disabler D andithe hangover time for which it will remain operated, are preferably each made'substantially equal to'T, where T is the overall transmis-' sion time of one one-way path of the fourwire circuit between terminals, although,as will be brought out later, under certain conditions this delay and hangover time maybe somewhat different. One way. of prodi'icing this delay and hangover is as indicated in the drawings by making-the relay 14 slow operating to a required degree and by making the relay 16 slow releasing to the required degree. v

The echo suppressor E8 may be effectively disabled in response to the operation of the disabler D by other means than those shown in Fig. 2. For example, the portionof the circuit contained between-the dot-dash lines AA and BB inFig. 2 may be modified as shown in Fig. 3 so that the relay 16 operates to open-circuit, instead of to short-circuit the output of the rectifier R by opening normal-c 1y closed contacts 23. This will serve to prevent a very sharp change in the transmission equivalent if the noise level is high on the transmission path WA.

The echo suppressor ES and the associated disablerl) at theeast end of the fourwire circuit may have constructions similar to'that of the echo suppressor ES and the disabler D described above so" as to operate in similar manner. For convenience, therefore, inthe following description of the operation of the whole system, the elements ES and D will be designated with the same characters as the corresponding elements' of ES and D but followed by a prime mark."

The operation of the system will now be described by reference to Figs- 1 and 2. It will be assumedthat speech waves for transmission from west to east are received over the two-wire line Lw by the transmission path EAat the west terminal of the four-wire circuit, and at that time no speech waves have as yet 'beenreceived at the east terminal from the two-wire line L for transmission from east to'west overthe transmission path W'A.

The received speech waves will be amplified by the amplifier A in the path EA, and

will 'be transmitted freely thereby, the gain of the amplifierA beingnormally adjusted so that there'is minimum loss inserted inthe path EA.

he main portion ofthe amplified west to east speech waves in the output of the amplifier A in the path EA. will be transmitted out over the path EA, which it will be assumed has an overall transmission time between the line sections L and L of substantially T seconds, to the east end of the four-wire circuit where it will be divided between the input of the amplifier A in the path EA and the input of the echo suppres sor ES The portion impressed on the input circuit of the amplifier A will be amplified thereby and then transmitted to the line section L over which it will be transmitted to the distant listening subscriber.

The portion of the west to east speech waves diverted into the input of the echo suppressor Eb will be amplified by the one'way amplifying device A therein and the amplified waves impressed upon the input of the full wave, vacuum tube rectifier R through the circuit for the push-pull stage of the amplifier A in the path VA causing a negative biasing potential to be continuously and smoothly applied to the grids of the tubes 7 and 8, which is directly proportional to the amplitude of the speech waves at the input of the echo suppressor ES.

The gain of the amplifier A will therefore be automatically adjusted so as to insert a loss in the input of the path VA which is directly proportional to the amplitude of the speech waves in the input of the echo suppressor ES which loss will be sufficient to insure that any east to west speech waves or any noise waves which may be thereafter transmitted out over the path VA to the west end of the four-wire circuit will not be of sutlicient amplitude to operate the echo suppressor E Meanwhile, a small portion of the amplified west to east speech waves in the output of the amplifier A in path EA will be diverted into the input of the disabler D These diverted speech waves will be amplified by the amplifier A and the amplified waves impressed by the transformer 13 on the input of the rectifier R, causing the operation of that rectifier to energize the windings of the relay 14 connected in its output circuit. The relay 14 will then operate to open its contacts 17, causing the energizing circuit for SlOW-OPCIfli-lhg relay 15 to be broken. Relay 15 will then release to close the normally open contacts 19 in the energizing circuit for the slow releasing relay 16, causing the windings of the relay 16 to be supplied with energizing current from the battery 20. Relay 16 will then operate to close the contacts 21, thereby connecting a short-circuiting connection across the output of the echo suppressor ES at the points 22 therein.

Because of the delay of T seconds introduced by the disabler D the short-circuiting of the output of the echo suppressor E8 at the points 22 will not occur until about the time the west to east speech waves which have been meanwhile transmitted out over the path EA have entered the input of the echo suppressor ES at the east end thereof and operated that echo suppressor to eflectively insert a loss proportional to the amplitude of the west to east speech current in the input of the transmission path WA.

From the above description of the system of Fig. 1 and 2, it is apparent that any east to west speech waves which may be sent out from the east terminal of the four-wire circuit over the path TVA more than T seconds after the west to east speech waves have been transmitted out over the path EA from the west terminal of the four-wire circuit, will not be of such amplitude as to cause operation of the disabler D at the east terminal or the echo suppressor E8 at the west terminal station because of the loss which has been previously effectively inserted in the input of the path VA in response to the west to east speech waves previously transmitted from the west terminal. For this condition, complete control of the switching circuits at both terminals has been obtained by the first talker associated with the west terminal station by the two-wire line IJyv.

Now, let it be supposed that the two subscribers at the opposite ends of the system both start talking about the same time, but that the talker at the east end of the system, that is, the talker associated with the two-wire line L starts talking just slightly before the talker at the westend of the system associated with the line L The cast talkers speech current will be amplified by the amplifier A in the transmission path VA. which is at normal maximum amplifying efliciency.

The main portion of the east talkers amplified speech currents will be transmitted out over the path WA towards the east terminal, and a smaller portion will be diverted into the input of the disabler D connected to the output of amplifier A in the path WA near the east terminal. Substantially T seconds later, where, according to the assumption made T is the overall. wave transmission time of the system between the two-wire line L and the two-wire line L over the path WA, the amplified waves transmitted over the path WA will cause the operation of the echo suppressor ES at the west terminal of the fourwire circuit to insert a loss in the transmission path EA proportional to the amplitude of the east to west speech currents, and the waves diverted into the input of the disabler D will cause the operation of the disabler D, at the east terminal of the four-wire circuit to disable the echo suppressor ES in its output.

Meanwhile, the later initiated west talkers speech currents will have been amplified by the amplifier A in the transmission path EA, which amplifier at that timeis in its normal operating condition, that is, in the condition to amplify with maximum eiiiciency. The

the main portion of the received west to east speech waves will be amplified by the amplifier A and the amplified waves in the output thereof impressed upon theeast line sec tion L and transmitted thereover to the listening subscriber associated therewith. Another portion of the west to east speech waves at the east terminal will be impressedon the input of the echo suppressor ES andwill cause the operation of the rectifier R therein. The rectified speech currents in the output of rectifier R will be impressed on the network 10. However, this will not be effective to change the gain of theamplifier A in the path WA because the output of the network 10 has been previously short-circuited atthe points 22 by operation of the disabler D in response to the west to east speech currents from the path EA.

Meanwhile the portion of the westto east speech currents diverted into the input of the disabler D at the west terminal will cause operation of that disabler to short-circuit the echo suppressor ES in its output but with a delay of T seconds. However, this will not effectively change conditions in the circuits because previous tothis time the input' currents to the echo suppressor ES due to the previous operation of suppressor ES at the east terminal are-so reduced in amplitude as to be unable to operate the suppressor E8 and the amplifier A in the. path EA will therefore remain at maximum transmitting efiiciency. I

[The circuits Wlll remain in the operated condition as just described until the'first the transmission paths on the operation of talkers speech currents cease or there is an appreciable pause therein such as would oc-v Due to the cur at theend of the sentence. slow release times of T-seconds of thedisablers D and D the short-circuiting of the echo suppressors will be maintained after the supply of speech currents to the disablers cease for a sufficient interval of time to pre vent false operation of the suppressors by.

echoesthat is, within a time interval substantially equal to' the overall transmissiontime over one side of the four-wire circuit between terminals, is that thefirsttalkers currents will be transmitted with maximum etficiency while only a portion of the speech of the second talker will get through to the first party. The latter may be of some advan tage on. long circuits from the standpoint of breaking in as will be brought out later.

The delay in operationof the disablers D and D at the two ends of the four-wire circuit should be preferably not less than T seconds in order that the first talker may have time to operate theecho suppressor at the opposite terminal before the second talker operates the suppressor'at the first talkers terminal. As stated above, it is preferable that this delay in operation of the two disabler circuits be substantially equal to T seconds, that is, to the overall transmission time of the four-wire circuit between terminals. The delay time however may be less than T seconds, in which case when the two talkers :at opposite ends of the-system both start talking within the overall transmission time of the cable, eachwill cause the operation of the disabler, at his own terminal to disable the echosuppressor controlledby the other talkers speech currents. Both echo suppressors will then remain ineflective while both parties continue talking, so that there'will be no suppression of echoes. This will result ina certain amount of double talking on each transmission path which will be more or less annoy ng, but not necessarily a serioushindrance to the carryingon of the conversation.

If the delay time of the two disablers D and D is greater thanT seconds, that is, greater than the overall transmission time of the four-wire circuit between terminals,

sidered less harmful than transmission lockouts, and, therefore, the design of the disablers D and D so that they have a delay time less than T is preferred to a design which would givethem a delay time greater than T.

In the abovedescription/nothing has been;

said about the effect of transmission noise-in the circuit. 'The .efiect of noise on the system of the invention will depend upon sion path EA or VA or in thetwo-way line section L or L Noise originating in the transmission pathsEA and WA of the fourwire circuits will merely cause the echo suppressors ES or. E55 to lntroduce a CGItLlII' amount of loss in the opposite transmission.

path. As soon as the disabler connected to that path is operated by speech, this loss is removed. The circuits should preferably be designed so that this loss is removed gradually in order to prevent very sharp changes in the transmission equivalents of the transmission path.

Noise originating in the two-wire line sec- .tion L or L will tend to operate the disablers D and D respectively. Ifthere is noise on both two-way line sections L and L both disablers D and D will be operated to disable the corresponding echo suppressors, and there will be no echo suppression. The disablers will be subject to noise from the twowire lines only, and there would be, therefore, effectually less noise to deal with than if the echo suppressors were located at the mid-point of the four-wire circuit. To avoid operation from noise, it is preferable that the disablers be set so that they are less sensitive than ordinary echo suppressors. This can be accomplished by the proper choice of the rectifier-s R R and the proper design of the relay circuits in the disablers. Reduction in the sensitivity of the disablers, of course, introduces the possibility of interruption of the talker by the listener. The chance of this occurring is not great as weak speech syl lables on the whole are also of short duration. The release time, or hangover, of the disablers could be set a little longer by proper design of the relay circuits therein so as to often hold over during the passing of short syllables, and thus to minimize the change of interruption of the talker by the listener.

A particular advantage of the circuits of the invention as just described is that, because the transmission efficiency of the transmission paths is merely lowered by the insertion of a loss therein in response to operation of the suppressor rather than reduced to zero by the open-circuiting or short-circuiting of these paths, one party by talking loud enough can always over-ride the loss produced by the other partys speech. It is seen therefore that the circuits of the invention are such as to facilitate break-ins.

In the preferred form of the invention which has been illustrated and described, chains of relays are utilized in the disablers D and D for producing the required delay in the operation thereof. It is within the scope of the invention to utilize a greater number or a lesser number of relays in the chains in the disablers for producing the required delay, or to utilize any other means for accomplishing the same result. For ex ample, electrical delay circuits may be used in place of some of the relays.

In the circuits of the invention which have been illustrated and described, echo suppressors of the type which operate to insert losses in a transmission path proportional to the am litude level of the waves in the opposite pat are employed. It is of course within the scope of the invention to utilize other types of echo suppressors well known in the art in placed the echo suppressors illustrated.

Other modifications which may be made in the circuits illustrated and described within the scope and spirit of the invention will be apparent to persons skilled in the art. The

invention is to be limited only within the scope of the appended claims.

What is claimed is:

1. A two-way signaling system comprising stations, two one-way transmission paths for transmitting signals in opposite directions between said stations, means connected to one of said one-way paths and responsive to wave transmission thereover for varying the transmission efficiency of the other of said one-way paths, and means responsive to transmission of signals in said other one-Way path for effectively disabling said transmission eificiency varying means within a time interval after the initiation of said signals in said other one-way path at least as small as the overall wave transmission time of one of said one-way paths between said stations.

2. A two-way signaling system comprising two one-Way transmission paths for transmitting signals in opposite directions between stations, means connected to one oneway path and responsive to wave transmission thereover for varying the transmission efiiciency of the other one-way path, and means responsive to transmission of signals in said other one-way path for effectively disabling said transmission efliciency disabling means at a time after the initiation of said signals in said other one-way path substantially equal to the overall wave transmission time of one of said one-way paths between said stations.

3. In combination with a two-way signaling system comprising one-way paths for transmitting signals in opposite directions between two-way line sections, means connected to each of said paths and responsive to the waves transmitted thereover for varying the transmission efliciency of the other oneway path near the input thereof, and means for preventing transmission lockouts caused by the simultaneous operation of both transmission eiiiciency varying means by signals initiated in both paths about the same time comprising means connected to each path near the input thereof and responsive to the transmission of signals therein for efl ectively disabling the transmission efiiciency varying means connected to the other path within a time interval after the initiation of said signals in the first path at least as small as the overall wave transmission time of said other one-way path between said two-way line sections.

4. In combination with a two-way signaling system comprising two one-Way transmission paths for transmitting signals in opposite directions between two-way line sections, means responsive to the waves transmitted over one of said one-way paths for varying the transmission efiiciency of the other one-way path in proportion to the amplitude level of said waves in said one one-way path, and means responsive to the V the overall transmission time of one'of said one-way paths between said two-way line sections.

5. In combination with a two-way signaling system comprising two oneway paths for transmitting signals in opposite directions between two-way line sections, means responsive to the waves transmitted over one of said one-way-paths for inserting. a loss in the other of said one-way paths varying in value in proportion to the amplitude level of said waves in said one one-way'path, and

means responsive to the transmission of signals over said other one-way path for effectively disabling ,said loss-inserting means within a time interval after the initiationof said signals in, said other one-way path at least as small as the overall transmission time of said one one-way path between said two-way line sections.

6. In combination with a two-way signaling system comprising two two-way line sections and two one-way transmission paths for transmitting signals in opposite directions between said two-way line sections, means for varying the transmitting eificiency of one of said paths in proportion to the amplitude of the waves transmitted over the other of said paths, and means responsive to the transmission of signals in said one path for effectively disabling said transmitting eificiency-varying means at a time after the initiation of said signals in said one path substantially equal to overall transmission time of said other one-way path betweensaid twoway line sections.

7. In a system for suppressing echoes in a two-way signaling circuit comprising two one-way transmission paths for transmitting signals in opposite directions between twoway line sections, means for continuously reducing the transmission efliciency of one of said one-way paths in proportion to the rise in amplitude lever of the waves transmitted over the other one-way path, and

7 means responsive to the transmission of mgnals in said one one-way path for effectively disabling said transmission eiiiciency reducing means at a time after the initiation of the signals in said one one-way path substantially equal to the over-all transmission time of one oneway path of the two-way circuit between said two-way line sections.

8. In a system for suppressing echoes in a two-way signal transmission circuit comprising one-way paths normally adapted for repeating signals in opposite directions between said two two-way line sections with maximum transmission eliiciency, means responsive to the waves transmitted over one of said one-way pathsfor continuously varying the transmitting etficieney of the other of i said one-way paths so that it is proportionatelyreduced when the amplitude level of said waves increase about a certain normal value,

and means responsive to the transmission of signals in said other of said paths for effec tively disabling said transmission elilciencyvarying means within a time interval after the initiation of said signals in said other path at least as small as'the overall. transmission time of said one one-way path between said two-way line sections.

9. In a system for suppressing echoes in a two-way signaling circuit comprising .two one-way transmission paths for transmitting signals in opposite directions between stations, each including an amplifier, means responsiveto the wave transmitted over one of said one-way paths for continuously varying the gain of the amplifier in the other of said paths in proportion of the amplitude level of said waves, and means responsive to the trans mission of signals over said other path for ellectively disabling said amplifier gain-varying means within a time interval after the initiation of said signals in said other path at least as small as the overall wave trans mission time of said one one-way pathrbe tween said stations.

10. In a system for suppressing echoes in a two-way signal transmission circuit comprising two one-way paths for transmitting signals in opposite directions between stations, each including a vacuum tube amplifying device having input and output electrodes, means responsive to the waves transmitted over one of said paths for continuously applying to the input electrodes of the amplifying device in the other of said paths a biasing potential varying in proportion to the amplitude level or" said waves, said biasing potential being such as to efiectively disable the amplifier when the amplitude level of the waves in said one one-way path exceeds a certain predetermined value, and means responsive to the transmission of signals in said other path for eliectively disabling said means for applying a biasing potential within a time interval after the initia-' tion of said signals in said other path at least as small as the overall wave transmission time plifying means in said other path when the amplitude level of said waves exceeds a predetermined value. and means responsive to the transmission of signals over said other one-way path for effectively disabling said amplifying efliciency-varying means at a time after the initiation of said signals in said other path substantially equivalent to the overall Wave transmission time of one oneway path of said f0ur-wire system between said two-way line sections.

In Witness whereof, I hereunto subscribe my name this 19th day of September, 1930. BJORN G. BJORNSON. 

